In recent years, new energy-production techniques have attracted attention from the standpoint of environmental issues, and among these techniques a fuel cell has attracted particular interest. The fuel cell converts chemical energy to electric energy through electrochemical reaction of hydrogen and oxygen, attaining high energy utilization efficiency. Therefore, practical studies have been carried out on realization of fuel cells for public use, industrial use, automobile use, etc.
Fuel cells are categorized in accordance with the type of employed electrolyte, and, among types, a phosphoric acid type, a molten carbonate type, a solid oxide type, and a solid polymer electrolyte type have been known. With regard to hydrogen sources, studies have been conducted on methanol; liquefied natural gas predominantly containing methane; city gas predominantly containing natural gas; a synthetic liquid fuel produced from natural gas serving as a feedstock; and petroleum-derived hydrocarbons such as naphtha and kerosene.
When hydrogen is produced from petroleum-derived hydrocarbons, the hydrocarbons are generally steam-reformed in the presence of catalysts. Among such catalysts, catalysts that contain ruthenium carried by a carrier as an active component have conventionally been studied, in view of their advantages; e.g., comparatively high activity and suppression of carbon deposition even under low steam/carbon ratio operational conditions. In recent years, these ruthenium catalysts have been expected to use in fuel cells, which require a long-life catalyst.
Since a co-catalyst effect of cerium oxide exerted on a ruthenium catalyst was discovered, ruthenium-cerium oxide-based catalysts have been studied and some patent applications have been filed (Japanese Patent Publication (kokoku) No. 59-29633 and Japanese Patent Application Laid-Open (kokai) Nos. 60-147242, 4-281845, 9-10586, and 2000-61307).
In addition to ruth nium-based catalysts, catalysts predominantly containing platinum, rhodium, palladium, iridium, or nickel have been studied. However, these catalysts have drawbacks, in that catalytic activity in terms of steam reforming of hydrocarbons remains unsatisfactory, and that carbon is deposited in a large amount during reforming.
In addition to the aforementioned steam reforming, other reforming processes for producing hydrogen, such as autothermal reforming, partial-oxidation reforming, and carbon dioxide reforming have been studied. As is known, all the above reforming processes can generally be performed through employment of the same reforming catalyst, and synthesis gas can also be produced through all the above processes with slight modification of reforming conditions. Studies have also been carried out on use of platinum group elements such as ruthenium, platinum, rhodium, palladium, and iridium in catalysts for the above autothermal reforming, partial-oxidation reforming, and carbon dioxide reforming. However, catalytic activity of the catalyst employing the elements remains unsatisfactory.